How do you measure chemicals for a 2600 gallon pool when the chemicals are measured for 10000 gallon pools or larger?

You must work out the fraction (size of your pool)/(size of pool on the chemical container) if it is given. If your pool is 2,600 galls and the amount of chemical is given for a 10,000 gall pool, you need 2600/10000 of the amount, ie 26/100, so for every 100 grams recommended, put in 26.